Many cells produce electrical impulses known as electrical activities (e.g., action potential) that propagate across their surface membrane. Action potentials travel quickly, and their arrival at a distant location initiates cellular processes such as the release of neurotransmitter molecules or the contraction of muscles (Hille B. Ion Channels of Excitable Membranes. Sinauer Associates, Inc. Sunderland, Mass., 2001). These electrical impulses are the means by which living cells transfer information over large distances in short time intervals.
Action potential theory contains two key elements (Hodgkin et al. J. Physiol. (Lond) 1952, 117:500-544). The first element is that the membrane of a cell can undergo transient changes in its selective permeability to, for example, Na+ and K+ ions. The second element is that the permeability changes depend on membrane voltage. These two elements create an interesting situation because selective permeability to ions determines the membrane voltage, while the voltage determines the permeability.
The family of protein molecules known as the voltage-dependent cation channels typically mediate electrical activity. This family includes potassium (K+), sodium (Na+) and calcium (Ca2+) selective members. The opening of a pore of a voltage-dependent ion channel, a process known as gating, is dependent upon the membrane voltage. When the pore of a voltage-dependent cation channel opens, it selectively conducts predominantly its namesake ion.
It is believed that charged amino acids, called gating charges, move through the membrane electric field before the pore opens, allowing membrane voltage to bias the equilibrium between closed and opened conformations (Armstrong et al. J. Gen. Physiol. 1974, 63:533-552; Sigworth et al. Q. Rev. Biophys. 1994, 27:1-40; and Bezanilla Physiol. Rev. 2000, 80:555-592).
In K+ channels, the gating charge per tetrameric channel corresponds to 12-14 electron charges (3.0-3.5 charges per subunit) crossing the entire membrane voltage difference. This large gating charge gives rise to a steep change in open probability as a function of membrane voltage.
All members of the voltage-dependent cation channel family typically contain six hydrophobic segments, S1 through S6 (S1-S6) (see FIGS. 1 and 2), per subunit. Four subunits (most often identical in K+ channels and linked together as homologous ‘domains’ in Na+ and Ca2+ channels) surround a central ion conduction pore. S5 through S6 line the pore and determine ion selectivity, while S1 through S4 form the voltage sensors. Certain charged amino acids within the voltage sensors account for most of the gating charge. These amino acids are particularly the first four arginines in S4.
Voltage-dependent ion channels are present in every cell and are involved in generation of electrical activity and information processing. As such, aberrant electrical activity can result in various conditions, such as heart arrhythmias, epilepsy, hypertension, etc.
There is a need for a composition and method for rapidly screening chemical compounds to determine whether the compounds bind to voltage-dependent ion channels.